Propane
expands when heated. That increases the pressure in
the tank. Above a certain pressure, the relief valve
opens to release a small amount of vapor. That release
keeps the pressure from building up too much inside
the tank. Once excess pressure has been released,
the relief valve closes automatically.

Its important to keep your tank painted with
a white, aluminum or other reflective color. A rusty,
unpainted or dark-colored tank may cause pressure
build-up resulting in a safety relief valve discharge.